Avoiding remembered-set maintenance overhead for memory segments known to be in a collection set

ABSTRACT

A garbage collector that employs the train algorithm to manage a generation in a computer system&#39;s dynamically allocated heap maintains for each of the generation&#39;s cars a respective remembered set that identifies all locations where references to objects in that car have been found by scanning locations identified by the mutator as having been modified. To avoid some of the expense of remembered-set updating, the collector refrains from attempting to add to a remembered set any reference located in a car that will be collected during the next collection increment. Additionally, if no mutator operation will occur before a collection set of one or more cars will be collected, any reference located outside that collection set but referring to an object within the collection set is not recorded in a remembered set but is recorded instead in a scratch-pad list of entries that identify references to collection-set objects that need to be evacuated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to memory management. It particularlyconcerns what has come to be known as “garbage collection.”

2. Background Information

In the field of computer systems, considerable effort has been expendedon the task of allocating memory to data objects. For the purposes ofthis discussion, the term object refers to a data structure representedin a computer system's memory. Other terms sometimes used for the sameconcept are record and structure. An object may be identified by areference, a relatively small amount of information that can be used toaccess the object. A reference can be represented as a “pointer” or a“machine address,” which may require, for instance, only sixteen,thirty-two, or sixty-four bits of information, although there are otherways to represent a reference.

In some systems, which are usually known as “object oriented,” objectsmay have associated methods, which are routines that can be invoked byreference to the object. They also may belong to a class, which is anorganizational entity that may contain method code or other informationshared by all objects belonging to that class. In the discussion thatfollows, though, the term object will not be limited to such structures;it will additionally include structures with which methods and classesare not associated.

The invention to be described below is applicable to systems thatallocate memory to objects dynamically. Not all systems employ dynamicallocation. In some computer languages, source programs must be sowritten that all objects to which the program's variables refer arebound to storage locations at compile time. This storage-allocationapproach, sometimes referred to as “static allocation,” is the policytraditionally used by the Fortran programming language, for example.

Even for compilers that are thought of as allocating objects onlystatically, of course, there is often a certain level of abstraction tothis binding of objects to storage locations. Consider the typicalcomputer system 10 depicted in FIG. 1, for example. Data, andinstructions for operating on them, that a microprocessor 11 uses mayreside in on-board cache memory or be received from further cache memory12, possibly through the mediation of a cache controller 13. Thatcontroller 13 can in turn receive such data from system read/writememory (“RAM”) 14 through a RAM controller 15 or from various peripheraldevices through a system bus 16. The memory space made available to anapplication program may be “virtual” in the sense that it may actuallybe considerably larger than RAM 14 provides. So the RAM contents will beswapped to and from a system disk 17.

Additionally, the actual physical operations performed to access some ofthe most-recently visited parts of the process's address space oftenwill actually be performed in the cache 12 or in a cache on boardmicroprocessor 11 rather than on the RAM 14, with which those cachesswap data and instructions just as RAM 14 and system disk 17 do witheach other.

A further level of abstraction results from the fact that an applicationwill often be run as one of many processes operating concurrently withthe support of an underlying operating system. As part of that system'smemory management, the application's memory space may be moved amongdifferent actual physical locations many times in order to allowdifferent processes to employ shared physical memory devices. That is,the location specified in the application's machine code may actuallyresult in different physical locations at different times because theoperating system adds different offsets to themachine-language-specified location.

Despite these expedients, the use of static memory allocation in writingcertain long-lived applications makes it difficult to restrict storagerequirements to the available memory space. Abiding by space limitationsis easier when the platform provides for dynamic memory allocation,i.e., when memory space to be allocated to a given object is determinedonly at run time.

Dynamic allocation has a number of advantages, among which is that therun-time system is able to adapt allocation to run-time conditions. Forexample, the programmer can specify that space should be allocated for agiven object only in response to a particular run-time condition. TheC-language library function malloc( ) is often used for this purpose.Conversely, the programmer can specify conditions under which memorypreviously allocated to a given object can be reclaimed for reuse. TheC-language library function free( ) results in such memory reclamation.

Because dynamic allocation provides for memory reuse, it facilitatesgeneration of large or long-lived applications, which over the course oftheir lifetimes may employ objects whose total memory requirements wouldgreatly exceed the available memory resources if they were bound tomemory locations statically.

Particularly for long-lived applications, though, allocation andreclamation of dynamic memory must be performed carefully. If theapplication fails to reclaim unused memory—or, worse, loses track of theaddress of a dynamically allocated segment of memory—its memoryrequirements will grow over time to exceed the system's availablememory. This kind of error is known as a “memory leak.”

Another kind of error occurs when an application reclaims memory forreuse even though it still maintains a reference to that memory. If thereclaimed memory is reallocated for a different purpose, the applicationmay inadvertently manipulate the same memory in multiple inconsistentways. This kind of error is known as a “dangling reference,” because anapplication should not retain a reference to a memory location once thatlocation is reclaimed. Explicit dynamic-memory management by usinginterfaces like malloc( )/free( ) often leads to these problems.

A way of reducing the likelihood of such leaks and related errors is toprovide memory-space reclamation in a more-automatic manner. Techniquesused by systems that reclaim memory space automatically are commonlyreferred to as “garbage collection.” Garbage collectors operate byreclaiming space that they no longer consider “reachable.” Staticallyallocated objects represented by a program's global variables arenormally considered reachable throughout a program's life. Such objectsare not ordinarily stored in the garbage collector's managed memoryspace, but they may contain references to dynamically allocated objectsthat are, and such objects are considered reachable. Clearly, an objectreferred to in the processor's call stack is reachable, as is an objectreferred to by register contents. And an object referred to by anyreachable object is also reachable.

The use of garbage collectors is advantageous because, whereas aprogrammer working on a particular sequence of code can perform his taskcreditably in most respects with only local knowledge of the applicationat any given time, memory allocation and reclamation require a globalknowledge of the program. Specifically, a programmer dealing with agiven sequence of code does tend to know whether some portion of memoryis still in use for that sequence of code, but it is considerably moredifficult for him to know what the rest of the application is doing withthat memory. By tracing references from some conservative notion of a“root set,” e.g., global variables, registers, and the call stack,automatic garbage collectors obtain global knowledge in a methodicalway. By using a garbage collector, the programmer is relieved of theneed to worry about the application's global state and can concentrateon local-state issues, which are more manageable. The result isapplications that are more robust, having no dangling references andfewer memory leaks.

Garbage-collection mechanisms can be implemented by various parts andlevels of a computing system. One approach is simply to provide them aspart of a batch compiler's output. Consider FIG. 2's simplebatch-compiler operation, for example. A computer system executes inaccordance with compiler object code and therefore acts as a compiler20. The compiler object code is typically stored on a medium such asFIG. 1's system disk 17 or some other machine-readable medium, and it isloaded into RAM 14 to configure the computer system to act as acompiler. In some cases, though, the compiler object code's persistentstorage may instead be provided in a server system remote from themachine that performs the compiling. The electrical signals that carrythe digital data by which the computer systems exchange that code areexamples of the kinds of electromagnetic signals by which the computerinstructions can be communicated. Others are radio waves, microwaves,and both visible and invisible light.

The input to the compiler is the application source code, and the endproduct of the compiler process is application object code. This objectcode defines an application 21, which typically operates on input suchas mouse clicks, etc., to generate a display or some other type ofoutput. This object code implements the relationship that the programmerintends to specify by his application source code. In one approach togarbage collection, the compiler 20, without the programmer's explicitdirection, additionally generates code that automatically reclaimsunreachable memory space.

Even in this simple case, though, there is a sense in which theapplication does not itself provide the entire garbage collector.Specifically, the application will typically call upon the underlyingoperating system's memory-allocation functions. And the operating systemmay in turn take advantage of various hardware that lends itselfparticularly to use in garbage collection. So even a very simple systemmay disperse the garbage-collection mechanism over a number ofcomputer-system layers.

To get some sense of the variety of system components that can be usedto implement garbage collection, consider FIG. 3's example of a morecomplex way in which various levels of source code can result in themachine instructions that a processor executes. In the FIG. 3arrangement, the human applications programmer produces source code 22written in a high-level language. A compiler 23 typically converts thatcode into “class files.” These files include routines written ininstructions, called “byte codes” 24, for a “virtual machine” thatvarious processors can be software-configured to emulate. Thisconversion into byte codes is almost always separated in time from thosecodes' execution, so FIG. 3 divides the sequence into a “compile-timeenvironment” 25 separate from a “run-time environment” 26, in whichexecution occurs. One example of a high-level language for whichcompilers are available to produce such virtual-machine instructions isthe Java™ programming language. (Java is a trademark or registeredtrademark of Sun Microsystems, Inc., in the United States and othercountries.)

Most typically, the class files' byte-code routines are executed by aprocessor under control of a virtual-machine process 27. That processemulates a virtual machine from whose instruction set the byte codes aredrawn. As is true of the compiler 23, the virtual-machine process 27 maybe specified by code stored on a local disk or some othermachine-readable medium from which it is read into FIG. 1's RAM 14 toconfigure the computer system to implement the garbage collector andotherwise act as a virtual machine. Again, though, that code'spersistent storage may instead be provided by a server system remotefrom the processor that implements the virtual machine, in which casethe code would be transmitted electrically or optically to thevirtual-machine-implementing processor.

In some implementations, much of the virtual machine's action inexecuting these byte codes is most like what those skilled in the artrefer to as “interpreting,” so FIG. 3 depicts the virtual machine asincluding an “interpreter” 28 for that purpose. In addition to orinstead of running an interpreter, many virtual-machine implementationsactually compile the byte codes concurrently with the resultant objectcode's execution, so FIG. 3 depicts the virtual machine as additionallyincluding a “just-in-time” compiler 29. We will refer to thejust-in-time compiler and the interpreter together as “executionengines” since they are the methods by which byte code can be executed.

Now, some of the functionality that source-language constructs specifycan be quite complicated, requiring many machine-language instructionsfor their implementation. One quite-common example is a source-languageinstruction that calls for 64-bit arithmetic on a 32-bit machine. Moregermane to the present invention is the operation of dynamicallyallocating space to a new object; the allocation of such objects must bemediated by the garbage collector.

In such situations, the compiler may produce “inline” code to accomplishthese operations. That is, all object-code instructions for carrying outa given source-code-prescribed operation will be repeated each time thesource code calls for the operation. But inlining runs the risk that“code bloat” will result if the operation is invoked at many source-codelocations.

The natural way of avoiding this result is instead to provide theoperation's implementation as a procedure, i.e., a single code sequencethat can be called from any location in the program. In the case ofcompilers, a collection of procedures for implementing many types ofsource-code-specified operations is called a runtime system for thelanguage. The execution engines and the runtime system of a virtualmachine are designed together so that the engines “know” whatruntime-system procedures are available in the virtual machine (and onthe target system if that system provides facilities that are directlyusable by an executing virtual-machine program.) So, for example, thejust-in-time compiler 29 may generate native code that includes calls tomemory-allocation procedures provided by the virtual machine's runtimesystem. These allocation routines may in turn invoke garbage-collectionroutines of the runtime system when there is not enough memory availableto satisfy an allocation. To represent this fact, FIG. 3 includes block30 to show that the compiler's output makes calls to the runtime systemas well as to the operating system 31, which consists of procedures thatare similarly system-resident but are not compiler-dependent.

Although the FIG. 3 arrangement is a popular one, it is by no meansuniversal, and many further implementation types can be expected.Proposals have even been made to implement the virtual machine 27'sbehavior in a hardware processor, in which case the hardware itselfwould provide some or all of the garbage-collection function.

The arrangement of FIG. 3 differs from FIG. 2 in that the compiler 23for converting the human programmer's code does not contribute toproviding the garbage-collection function; that results largely from thevirtual machine 27's operation. Those skilled in that art will recognizethat both of these organizations are merely exemplary, and many modernsystems employ hybrid mechanisms, which partake of the characteristicsof traditional compilers and traditional interpreters both.

The invention to be described below is applicable independently ofwhether a batch compiler, a just-in-time compiler, an interpreter, orsome hybrid is employed to process source code. In the remainder of thisapplication, therefore, we will use the term compiler to refer to anysuch mechanism, even if it is what would more typically be called aninterpreter.

In short, garbage collectors can be implemented in a wide range ofcombinations of hardware and/or software. As is true of most of thegarbage-collection techniques described in the literature, the inventionto be described below is applicable to most such systems.

By implementing garbage collection, a computer system can greatly reducethe occurrence of memory leaks and other software deficiencies in whichhuman programming frequently results. But it can also have significantadverse performance effects if it is not implemented carefully. Todistinguish the part of the program that does “useful” work from thatwhich does the garbage collection, the term mutator is sometimes used indiscussions of these effects; from the collector's point of view, whatthe mutator does is mutate active data structures' connectivity.

Some garbage-collection approaches rely heavily on interleavinggarbage-collection steps among mutator steps. In one type ofgarbage-collection approach, for instance, the mutator operation ofwriting a reference is followed immediately by garbage-collector stepsused to maintain a reference count in that object's header, and code forsubsequent new-object storage includes steps for finding space occupiedby objects whose reference count has fallen to zero. Obviously, such anapproach can slow mutator operation significantly.

Other approaches therefore interleave very few garbage-collector-relatedinstructions into the main mutator process but instead interrupt it fromtime to time to perform garbage-collection cycles, in which the garbagecollector finds unreachable objects and reclaims their memory space forreuse. Such an approach will be assumed in discussing FIG. 4's depictionof a simple garbage-collection operation. Within the memory spaceallocated to a given application is a part 40 managed by automaticgarbage collection. In the following discussion, this will be referredto as the “heap,” although in other contexts that term refers to alldynamically allocated memory. During the course of the application'sexecution, space is allocated for various objects 42, 44, 46, 48, and50. Typically, the mutator allocates space within the heap by invokingthe garbage collector, which at some level manages access to the heap.Basically, the mutator asks the garbage collector for a pointer to aheap region where it can safely place the object's data. The garbagecollector keeps track of the fact that the thus-allocated region isoccupied. It will refrain from allocating that region in response to anyother request until it determines that the mutator no longer needs theregion allocated to that object.

Garbage collectors vary as to which objects they consider reachable andunreachable. For the present discussion, though, an object will beconsidered “reachable” if it is referred to, as object 42 is, by areference in the root set 52. The root set consists of reference valuesstored in the mutator's threads' call stacks, the CPU registers, andglobal variables outside the garbage-collected heap. An object is alsoreachable if it is referred to, as object 46 is, by another reachableobject (in this case, object 42). Objects that are not reachable can nolonger affect the program, so it is safe to re-allocate the memoryspaces that they occupy.

A typical approach to garbage collection is therefore to identify allreachable objects and reclaim any previously allocated memory that thereachable objects do not occupy. A typical garbage collector mayidentify reachable objects by tracing references from the root set 52.For the sake of simplicity, FIG. 4 depicts only one reference from theroot set 52 into the heap 40. (Those skilled in the art will recognizethat there are many ways to identify references, or at least datacontents that may be references.) The collector notes that the root setpoints to object 42, which is therefore reachable, and that reachableobject 42 points to object 46, which therefore is also reachable. Butthose reachable objects point to no other objects, so objects 44, 48,and 50 are all unreachable, and their memory space may be reclaimed.This may involve, say, placing that memory space in a list of freememory blocks.

To avoid excessive heap fragmentation, some garbage collectorsadditionally relocate reachable objects. FIG. 5 shows a typicalapproach. The heap is partitioned into two halves, hereafter called“semi-spaces.” For one garbage-collection cycle, all objects areallocated in one semi-space 54, leaving the other semi-space 56 free.When the garbage-collection cycle occurs, objects identified asreachable are “evacuated” to the other semi-space 56, so all ofsemi-space 54 is then considered free. Once the garbage-collection cyclehas occurred, all new objects are allocated in the lower semi-space 56until yet another garbage-collection cycle occurs, at which time thereachable objects are evacuated back to the upper semi-space 54.

Although this relocation requires the extra steps of copying thereachable objects and updating references to them, it tends to be quiteefficient, since most new objects quickly become unreachable, so most ofthe current semi-space is actually garbage. That is, only a relativelyfew, reachable objects need to be relocated, after which the entiresemi-space contains only garbage and can be pronounced free forreallocation.

Now, a collection cycle can involve following all reference chains fromthe basic root set—i.e., from inherently reachable locations such as thecall stacks, class statics and other global variables, and registers-andreclaiming all space occupied by objects not encountered in the process.And the simplest way of performing such a cycle is to interrupt themutator to provide a collector interval in which the entire cycle isperformed before the mutator resumes. For certain types of applications,this approach to collection-cycle scheduling is acceptable and, in fact,highly efficient.

For many interactive and real-time applications, though, this approachis not acceptable. The delay in mutator operation that the collectioncycle's execution causes can be annoying to a user and can prevent areal-time application from responding to its environment with therequired speed. In some applications, choosing collection timesopportunistically can reduce this effect. Collection intervals can beinserted when an interactive mutator reaches a point at which it awaitsuser input, for instance.

So it may often be true that the garbage-collection operation's effecton performance can depend less on the total collection time than on whencollections actually occur. But another factor that often is even moredeterminative is the duration of any single collection interval, i.e.,how long the mutator must remain quiescent at any one time. In aninteractive system, for instance, a user may never noticehundred-millisecond interuptions for garbage collection, whereas mostusers would find interruptions lasting for two seconds to be annoying.

The cycle may therefore be divided up among a plurality of collectorintervals. When a collection cycle is divided up among a plurality ofcollection intervals, it is only after a number of intervals that thecollector will have followed all reference chains and be able toidentify as garbage any objects not thereby reached. This approach ismore complex than completing the cycle in a single collection interval;the mutator will usually modify references between collection intervals,so the collector must repeatedly update its view of the reference graphin the midst of the collection cycle. To make such updates practical,the mutator must communicate with the collector to let it know whatreference changes are made between intervals.

An even more complex approach, which some systems use to eliminatediscrete pauses or maximize resource-use efficiency, is to execute themutator and collector in concurrent execution threads. Most systems thatuse this approach use it for most but not all of the collection cycle;the mutator is usually interrupted for a short collector interval, inwhich a part of the collector cycle takes place without mutation.

Independent of whether the collection cycle is performed concurrentlywith mutator operation, is completed in a single interval, or extendsover multiple intervals is the question of whether the cycle iscomplete, as has tacitly been assumed so far, or is instead“incremental.” In incremental collection, a collection cycle constitutesonly an increment of collection: the collector does not follow allreference chains from the basic root set completely. Instead, itconcentrates on only a portion, or collection set, of the heap.Specifically, it identifies every collection-set object referred to by areference chain that extends into the collection set from outside of it,and it reclaims the collection-set space not occupied by such objects,possibly after evacuating them from the collection set.

By thus culling objects referenced by reference chains that do notnecessarily originate in the basic root set, the collector can bethought of as expanding the root set to include as roots some locationsthat may not be reachable. Although incremental collection therebyleaves “floating garbage,” it can result in relatively low pause timeseven if entire collection increments are completed during respectivesingle collection intervals.

Most collectors that employ incremental collection operate in“generations” although this is not necessary in principle. Differentportions, or generations, of the heap are subject to differentcollection policies. New objects are allocated in a “young” generation,and older objects are promoted from younger generations to older or more“mature” generations. Collecting the younger generations more frequentlythan the others yields greater efficiency because the youngergenerations tend to accumulate garbage faster; newly allocated objectstend to “die,” while older objects tend to “survive.”

But generational collection greatly increases what is effectively theroot set for a given generation. Consider FIG. 6, which depicts a heapas organized into three generations 58, 60, and 62. Assume thatgeneration 60 is to be collected. The process for this individualgeneration may be more or less the same as that described in connectionwith FIGS. 4 and 5 for the entire heap, with one major exception. In thecase of a single generation, the root set must be considered to includenot only the call stack, registers, and global variables represented byset 52 but also objects in the other generations 58 and 62, whichthemselves may contain references to objects in generation 60. Sopointers must be traced not only from the basic root set 52 but alsofrom objects within the other generations.

One could perform this tracing by simply inspecting all references inall other generations at the beginning of every collection interval, andit turns out that this approach is actually feasible in some situations.But it takes too long in other situations, so workers in this field haveemployed a number of approaches to expediting reference tracing. Oneapproach is to include so-called write barriers in the mutator process.A write barrier is code added to a write operation to record informationfrom which the collector can determine where references were written ormay have been since the last collection interval. A reference list canthen be maintained by taking such a list as it existed at the end of theprevious collection interval and updating it by inspecting onlylocations identified by the write barrier as possibly modified since thelast collection interval.

One of the many write-barrier implementations commonly used by workersin this art employs what has been referred to as the “card table.” FIG.6 depicts the various generations as being divided into smallersections, known for this purpose as “cards.” Card tables 64, 66, and 68associated with respective generations contain an entry for each oftheir cards. When the mutator writes a reference in a card, it makes anappropriate entry in the card-table location associated with that card(or, say, with the card in which the object containing the referencebegins). Most write-barrier implementations simply make a Boolean entryindicating that the write operation has been performed, although somemay be more elaborate. The mutator having thus left a record of wherenew or modified references may be, the collector can thereafter prepareappropriate summaries of that information, as will be explained in duecourse. For the sake of concreteness, we will assume that the summariesare maintained by steps that occur principally at the beginning of eachcollection interval.

Of course, there are other write-barrier approaches, such as simplyhaving the write barrier add to a list of addresses where referenceswhere written. Also, although there is no reason in principle to favorany particular number of generations, and although FIG. 6 shows three,most generational garbage collectors have only two generations, of whichone is the young generation and the other is the mature generation.Moreover, although FIG. 6 shows the generations as being of the samesize, a more-typical configuration is for the young generation to beconsiderably smaller. Finally, although we assumed for the sake ofsimplicity that collection during a given interval was limited to onlyone generation, a more-typical approach is actually to collect the wholeyoung generation at every interval but to collect the mature one lessfrequently.

Some collectors collect the entire young generation in every intervaland may thereafter perform mature-generation collection in the sameinterval. It may therefore take relatively little time to scan allyoung-generation objects remaining after young-generation collection tofind references into the mature generation. Even when such collectors douse card tables, therefore, they often do not use them for findingyoung-generation references that refer to mature-generation objects. Onthe other hand, laboriously scanning the entire mature generation forreferences to young-generation (or mature-generation) objects wouldordinarily take too long, so the collector uses the card table to limitthe amount of memory it searches for mature-generation references.

Now, although it typically takes very little time to collect the younggeneration, it may take more time than is acceptable within a singlegarbage-collection interval to collect the entire mature generation. Sosome garbage collectors may collect the mature generation incrementally;that is, they may perform only a part of the mature generation'scollection during any particular collection cycle. Incrementalcollection presents the problem that, since the generation's unreachableobjects outside the “collection set” of objects processed during thatcycle cannot be recognized as unreachable, collection-set objects towhich they refer tend not to be, either.

To reduce the adverse effect this would otherwise have on collectionefficiency, workers in this field have employed the “train algorithm,”which FIG. 7 depicts. A generation to be collected incrementally isdivided into sections, which for reasons about to be described arereferred to as “car sections.” Conventionally, a generation'sincremental collection occurs in fixed-size sections, and a carsection's size is that of the generation portion to be collected duringone cycle.

The discussion that follows will occasionally employ the nomenclature inthe literature by using the term car instead of car section. But theliterature seems to use that term to refer variously not only to memorysections themselves but also to data structures that the train algorithmemploys to manage them when they contain objects, as well as to themore-abstract concept that the car section and managing data structurerepresent in discussions of the algorithm. So the following discussionwill more frequently use the expression car section to emphasize theactual sections of memory space for whose management the car concept isemployed.

According to the train algorithm, the car sections are grouped into“trains,” which are ordered, conventionally according to age. Forexample, FIG. 7 shows an oldest train 73 consisting of a generation 74'sthree car sections described by associated data structures 75, 76, and78, while a second train 80 consists only of a single car section,represented by structure 82, and the youngest train 84 (referred to asthe “allocation train”) consists of car sections that data structures 86and 88 represent. As will be seen below, car sections' train membershipscan change, and any car section added to a train is typically added tothe end of a train.

Conventionally, the car collected in an increment is the one addedearliest to the oldest train, which in this case is car 75. All of thegeneration's cars can thus be thought of as waiting for collection in asingle long line, in which cars are ordered in accordance with the orderof the trains to which they belong and, within trains, in accordancewith the order in which they were added to those trains.

As is usual, the way in which reachable objects are identified is todetermine whether there are references to them in the root set or in anyother object already determined to be reachable. In accordance with thetrain algorithm, the collector additionally performs a test to determinewhether there are any references at all from outside the oldest train toobjects within it. If there are not, then all cars within the train canbe reclaimed, even though not all of those cars are in the collectionset. And the train algorithm so operates that inter-car references tendto be grouped into trains, as will now be explained.

To identify references into the car from outside of it, train-algorithmimplementations typically employ “remembered sets.” As card tables are,remembered sets are used to keep track of references. Whereas acard-table entry contains information about references that theassociated card contains, though, a remembered set associated with agiven region contains information about references into that region fromlocations outside of it. In the case of the train algorithm, rememberedsets are associated with car sections. Each remembered set, such as car75's remembered set 90, lists locations in the generation that containreferences into the associated car section.

The remembered sets for all of a generation's cars are typically updatedat the start of each collection interval. To illustrate how suchupdating and other collection operations may be carried out, FIG. 8depicts an operational sequence in a system of the typical typementioned above. That is, it shows a sequence of operations that mayoccur in a system in which the entire garbage-collected heap is dividedinto two generations, namely, a young generation and an old generation,and in which the young generation is much smaller than the oldgeneration. FIG. 8 is also based on the assumption and that the trainalgorithm is used only for collecting the old generation.

Block 102 represents a period of the mutator's operation. As wasexplained above, the mutator makes a card-table entry to identify anycard that it has “dirtied” by adding or modifying a reference that thecard contains. At some point, the mutator will be interrupted forcollector operation. Different implementations employ different eventsto trigger such an interruption, but we will assume for the sake ofconcreteness that the system's dynamic-allocation routine causes suchinterruptions when no room is left in the young generation for anyfurther allocation. A dashed line 103 represents the transition frommutator operation and collector operation.

In the system assumed for the FIG. 8 example, the collector collects the(entire) young generation each time such an interruption occurs. Whenthe young generation's collection ends, the mutator operation usuallyresumes, without the collector's having collected any part of the oldgeneration. Once in a while, though, the collector also collects part ofthe old generation, and FIG. 8 is intended to illustrate such anoccasion.

When the collector's interval first starts, it first processes the cardtable, in an operation that block 104 represents. As was mentionedabove, the collector scans the “dirtied” cards for references into theyoung generation. If a reference is found, that fact is memorializedappropriately. If the reference refers to a young-generation object, forexample, an expanded card table may be used for this purpose. For eachcard, such an expanded card table might include a multi-byte array usedto summarize the card's reference contents. The summary may, forinstance, be a list of offsets that indicate the exact locations withinthe card of references to young-generation objects, or it may be a listof fine-granularity “sub-cards” within which references toyoung-generation objects may be found. If the reference refers to anold-generation object, the collector often adds an entry to theremembered set associated with the car containing that old-generationobject. The entry identifies the reference's location, or at least asmall region in which the reference can be found. For reasons that willbecome apparent, though, the collector will typically not bother toplace in the remembered set the locations of references from objects incar sections farther forward in the collection queue than thereferred-to object, i.e., from objects in older trains or in cars addedearlier to the same train.

The collector then collects the young generation, as block 105indicates. (Actually, young-generation collection may be interleavedwith the dirty-region scanning, but the drawing illustrates it forpurpose of explanation as being separate.) If a young-generation objectis referred to by a reference that card-table scanning has revealed,that object is considered to be potentially reachable, as is anyyoung-generation object referred to by a reference in the root set or inanother reachable young-generation object. The space occupied by anyyoung-generation object thus considered reachable is withheld fromreclamation. For example, it may be evacuated to a young-generationsemi-space that will be used for allocation during the next mutatorinterval. It may instead be promoted into the older generation, where itis placed into a car containing a reference to it or into a car in thelast train. Or some other technique may be used to keep the memory spaceit occupies off the system's free list. The collector then reclaims anyyoung-generation space occupied by any other objects, i.e., by anyyoung-generation objects not identified as transitively reachablethrough references located outside the young generation.

The collector then performs the train algorithm's central test, referredto above, of determining whether there are any references into theoldest train from outside of it. As was mentioned above, the actualprocess of determining, for each object, whether it can be identified asunreachable is performed for only a single car section in any cycle. Inthe absence of features such as those provided by the train algorithm,this would present a problem, because garbage structures may be largerthan a car section. Objects in such structures would therefore(erroneously) appear reachable, since they are referred to from outsidethe car section under consideration. But the train algorithmadditionally keeps track of whether there are any references into agiven car from outside the train to which it belongs, and trains' sizesare not limited. As will be apparent presently, objects not found to beunreachable are relocated in such a way that garbage structures tend tobe gathered into respective trains into which, eventually, no referencesfrom outside the train point. If no references from outside the trainpoint to any objects inside the train, the train can be recognized ascontaining only garbage. This is the test that block 106 represents. Allcars in a train thus identified as containing only garbage can bereclaimed.

The question of whether old-generation references point into the trainfrom outside of it is (conservatively) answered in the course ofupdating remembered sets; in the course of updating a car's rememberedset, it is a simple matter to flag the car as being referred to fromoutside the train. The step-106 test additionally involves determiningwhether any references from outside the old generation point into theoldest train. Various approaches to making this determination have beensuggested, including the conceptually simple approach of merelyfollowing all reference chains from the root set until those chains (1)terminate, (2) reach an old-generation object outside the oldest train,or (3) reach an object in the oldest train. In the two-generationexample, most of this work can be done readily by identifying referencesinto the collection set from live young-generation objects during theyoung-generation collection. If one or more such chains reach the oldesttrain, that train includes reachable objects. It may also includereachable objects if the remembered-set-update operation has found oneor more references into the oldest train from outside of it. Otherwise,that train contains only garbage, and the collector reclaims all of itscar sections for reuse, as block 107 indicates. The collector may thenreturn control to the mutator, which resumes execution, as FIG. 8B'sblock 108 indicates.

If the train contains reachable objects, on the other hand, thecollector turns to evacuating potentially reachable objects from thecollection set. The first operation, which block 110 represents, is toremove from the collection set any object that is reachable from theroot set by way of a reference chain that does not pass through the partof the old generation that is outside of the collection set. In theillustrated arrangement, in which there are only two generations, andthe young generation has previously been completely collected during thesame interval, this means evacuating from a collection set any objectthat (1) is directly referred to by a reference in the root set, (2) isdirectly referred to by a reference in the young generation (in which noremaining objects have been found unreachable), or (3) is referred to byany reference in an object thereby evacuated. All of the objects thusevacuated are placed in cars in the youngest train, which was newlycreated during the collection cycle. Certain of the mechanics involvedin the evacuation process are described in more detail in connectionwith similar evacuation performed, as blocks 112 and 114 indicate, inresponse to remembered-set entries.

FIG. 9 illustrates how the processing represented by block 114 proceeds.The entries identify heap regions, and, as block 116 indicates, thecollector scans the thus-identified heap regions to find references tolocations in the collectionset. As blocks 118 and 120 indicate, thatentry's processing continues until the collector finds no more suchreferences. Every time the collector does find such a reference, itchecks to determine whether, as a result of a previous entry'sprocessing, the referred-to object has already been evacuated. If it hasnot, the collector evacuates the referred-to object to a (possibly new)car in the train containing the reference, as blocks 122 and 124indicate.

As FIG. 10 indicates, the evacuation operation includes more than justobject relocation, which block 126 represents. Once the object has beenmoved, the collector places a forwarding pointer in the collection-setlocation from which it was evacuated, for a purpose that will becomeapparent presently. Block 128 represents that step. (Actually, there aresome cases in which the evacuation is only a “logical” evacuation: thecar containing the object is simply re-linked to a different logicalplace in the collection sequence, but its address does not change. Insuch cases, forwarding pointers are unnecessary.) Additionally, thereference in response to which the object was evacuated is updated topoint to the evacuated object's new location, as block 130 indicates.And, as block 132 indicates, any reference contained in the evacuatedobject is processed, in an operation that FIGS. 11A and 11B (“FIG. 11”)depicts.

For each one of the evacuated object's references, the collector checksto see whether the location that it refers to is in the collection set.As blocks 134 and 136 indicate, the reference processing continues untilall references in the evacuated object have been processed. In themeantime, if a reference refers to a collection-set location thatcontains an object not yet evacuated, the collector evacuates thereferred-to object to the train to which the evacuated object containingthe reference was evacuated, as blocks 138 and 140 indicate.

If the reference refers to a location in the collection set from whichthe object has already been evacuated, then the collector uses theforwarding pointer left in that location to update the reference, asblock 142 indicates. The remembered set of the referred-to object's carwill have an entry that identifies the evacuated object's old locationas one containing a reference to the referred-to object. But theevacuation has placed the reference in a new location, for which theremembered set of the referred-to object's car may not have an entry.So, if that new location is not as far forward as the referred-toobject, the collector adds to that remembered set an entry identifyingthe reference's new region, as blocks 144 and 146 indicate. As thedrawing indicates, the remembered set may similarly need to be updatedeven if the referred-to object is not in the collection set.

Now, some train-algorithm implementations postpone processing of thereferences contained in evacuated collection-set objects until after alldirectly reachable collection-set objects have been evacuated. In theimplementation that FIG. 10 illustrates, though, the processing of agiven evacuated object's references occurs before the next object isevacuated. So FIGS. 11's blocks 134 and 148 indicate that the FIG. 11operation is completed when all of the references contained in theevacuated object have been processed. This completes FIG. 10'sobject-evacuation operation, which FIG. 9's block 124 represents.

As FIG. 9 indicates, each collection-set object referred to by areference in a remembered-set-entry-identified location is thusevacuated if it has not been already. If the object has already beenevacuated from the referred-to location, the reference to that locationis updated to point to the location to which the object has beenevacuated. If the remembered set associated with the car containing theevacuated object's new location does not include an entry for thereference's location, it is updated to do so if the car containing thereference is younger than the car containing the evacuated object. Block150 represents updating the reference and, if necessary, the rememberedset.

As FIG. 8's blocks 112 and 114 indicate, this processing ofcollection-set remembered set is performed initially only for entriesthat do not refer to locations in the oldest train. Those that do areprocessed only after all others have been, as blocks 152 and 154indicate.

When this process has been completed, the collection set's memory spacecan be reclaimed, as block 164 indicates, since no remaining object isreferred to from outside the collection set: any remainingcollection-set object is unreachable. The collector then relinquishescontrol to the mutator.

FIGS. 12A–12J illustrate results of using the train algorithm. FIG. 12Arepresents a generation in which objects have been allocated in nine carsections. The oldest train has four cars, numbered 1.1 through 1.4. Car1.1 has two objects, A and B. There is a reference to object B in theroot set (which, as was explained above, includes live objects in theother generations). Object A is referred to by object L, which is in thethird train's sole car section. In the generation's remembered sets 170,a reference in object L has therefore been recorded against car 1.1.

Processing always starts with the oldest train's earliest-added car, sothe garbage collector refers to car 1.1's remembered set and finds thatthere is a reference from object L into the car being processed. Itaccordingly evacuates object A to the train that object L occupies. Theobject being evacuated is often placed in one of the selected train'sexisting cars, but we will assume for present purposes that there is notenough room. So the garbage collector evacuates object A into a new carsection and updates appropriate data structures to identify it as thenext car in the third train. FIG. 12B depicts the result: a new car hasbeen added to the third train, and object A is placed in it.

FIG. 12B also shows that object B has been evacuated to a new caroutside the first train. This is because object B has an externalreference, which, like the reference to object A, is a reference fromoutside the first train, and one goal of the processing is to formtrains into which there are no further references. Note that, tomaintain a reference to the same object, object L's reference to objectA has had to be rewritten, and so have object B's reference to object Aand the inter-generational pointer to object B. In the illustratedexample, the garbage collector begins a new train for the car into whichobject B is evacuated, but this is not a necessary requirement of thetrain algorithm. That algorithm requires only that externally referencedobjects be evacuated to a newer train.

Since car 1.1 no longer contains live objects, it can be reclaimed, asFIG. 12B also indicates. Also note that the remembered set for car 2.1now includes the address of a reference in object A, whereas it did notbefore. As was stated before, remembered sets in the illustratedembodiment include only references from cars further back in the orderthan the one with which the remembered set is associated. The reason forthis is that any other cars will already be reclaimed by the time thecar associated with that remembered set is processed, so there is noreason to keep track of references from them.

The next step is to process the next car, the one whose index is 1.2.Conventionally, this would not occur until some collection cycle afterthe one during which car 1.1 is collected. For the sake of simplicity wewill assume that the mutator has not changed any references into thegeneration in the interim.

FIG. 12B depicts car 1.2 as containing only a single object, object C,and that car's remembered set contains the address of an inter-carreference from object F. The garbage collector follows that reference toobject C. Since this identifies object C as possibly reachable, thegarbage collector evacuates it from car set 1.2, which is to bereclaimed. Specifically, the garbage collector removes object C to a newcar section, section 1.5, which is linked to the train to which thereferring object F's car belongs. Of course, object F's reference needsto be updated to object C's new location. FIG. 12C depicts theevacuation's result.

FIG. 12C also indicates that car set 1.2 has been reclaimed, and car 1.3is next to be processed. The only address in car 1.3's remembered set isthat of a reference in object G. Inspection of that reference revealsthat it refers to object F. Object F may therefore be reachable, so itmust be evacuated before car section 1.3 is reclaimed. On the otherhand, there are no references to objects D and E, so they are clearlygarbage. FIG. 12D depicts the result of reclaiming car 1.3's space afterevacuating possibly reachable object F.

In the state that FIG. 12D depicts, car 1.4 is next to be processed, andits remembered set contains the addresses of references in objects K andC. Inspection of object K's reference reveals that it refers to objectH, so object H must be evacuated. Inspection of the other remembered-setentry, the reference in object C, reveals that it refers to object G, sothat object is evacuated, too. As FIG. 12E illustrates, object H must beadded to the second train, to which its referring object K belongs. Inthis case there is room enough in car 2.2, which its referring object Koccupies, so evacuation of object H does not require that object K'sreference to object H be added to car 2.2's remembered set. Object G isevacuated to a new car in the same train, since that train is wherereferring object C resides. And the address of the reference in object Gto object C is added to car 1.5's remembered set.

FIG. 12E shows that this processing has eliminated all references intothe first train, and it is an important part of the train algorithm totest for this condition. That is, even though there are references intoboth of the train's cars, those cars' contents can be recognized as allgarbage because there are no references into the train from outside ofit. So all of the first train's cars are reclaimed.

The collector accordingly processes car 2.1 during the next collectioncycle, and that car's remembered set indicates that there are tworeferences outside the car that refer to objects within it. Thosereferences are in object K, which is in the same train, and object A,which is not. Inspection of those references reveals that they refer toobjects I and J, which are evacuated.

The result, depicted in FIG. 12F, is that the remembered sets for thecars in the second train reveal no inter-car references, and there areno inter-generational references into it, either. That train's carsections therefore contain only garbage, and their memory space can bereclaimed.

So car 3.1 is processed next. Its sole object, object L, is referred tointer-generationally as well as by a reference in the fourth train'sobject M. As FIG. 12G shows, object L is therefore evacuated to thefourth train. And the address of the reference in object L to object Ais placed in the remembered set associated with car 3.2, in which objectA resides.

The next car to be processed is car 3.2, whose remembered set includesthe addresses of references into it from objects B and L. Inspection ofthe reference from object B reveals that it refers to object A, whichmust therefore be evacuated to the fifth train before car 3.2 can bereclaimed. Also, we assume that object A cannot fit in car section 5.1,so a new car 5.2 is added to that train, as FIG. 12H shows, and object Ais placed in its car section. All referred-to objects in the third trainhaving been evacuated, that (single-car) train can be reclaimed in itsentirety.

A further observation needs to be made before we leave FIG. 12G. Car3.2's remembered set additionally lists a reference in object L, so thegarbage collector inspects that reference and finds that it points tothe location previously occupied by object A. This brings up a featureof copying-collection techniques such as the typical train-algorithmimplementation. When the garbage collector evacuates an object from acar section, it marks the location as having been evacuated and leavesthe address of the object's new location. So, when the garbage collectortraces the reference from object L, it finds that object A has beenremoved, and it accordingly copies the new location into object L as thenew value of its reference to object A.

In the state that FIG. 12H illustrates, car 4.1 is the next to beprocessed. Inspection of the fourth train's remembered sets reveals nointer-train references into it, but the inter-generational scan(possibly performed with the aid of FIG. 6's card tables) revealsinter-generational references into car 4.2. So the fourth train cannotbe reclaimed yet. The garbage collector accordingly evacuates car 4.1'sreferred-to objects in the normal manner, with the result that FIG. 121depicts.

In that state, the next car to be processed has only inter-generationalreferences into it. So, although its referred-to objects must thereforebe evacuated from the train, they cannot be placed into trains thatcontain references to them. Conventionally, such objects are evacuatedto a train at the end of the train sequence. In the illustratedimplementation, a new train is formed for this purpose, so the result ofcar 4.2's processing is the state that FIG. 12J depicts.

Processing continues in this same fashion. Of course, subsequentcollection cycles will not in general proceed, as in the illustratedcycles, without any reference changes by the mutator and without anyaddition of further objects. But reflection reveals that the generalapproach just described still applies when such mutations occur.

Although automatic garbage collection tends to make programs morereliable, it can also slow their execution, so it is important to makethe garbage-collection operations as efficient as possible. Among theways of doing so is the above-mentioned use of remembered sets toexpedite finding references to collection-set objects. Remembered-setuse is beneficial not only in the train algorithm but also in othertechniques for incremental collection; other techniques, too, can dividememory into segments for which it maintains remembered sets. Butremembered-set maintenance itself imposes a cost. Adding aremembered-set entry is time-consuming because, among other things, itincludes testing for relative car location checking for duplicateentries.

SUMMARY OF THE INVENTION

I have found ways to reduce the expense of the updating operation in thetrain algorithm and other incremental-collection techniques. One wayinvolves, before the updating operation occurs, having the collectoridentify at least part of what will become the next collectionincrement's collection set. Then, as the collector scans themutator-modified regions for references, it omits the attendantremembered-set updating in those cases in which the references therebyfound are located in the identified part of the collection set.

I have recognized that doing so does not compromise the operation offinding reachable objects. If the reference whose location is therebyomitted from a remembered set is not in a reference chain by which anobject in the collection set is reachable, no harm is done. If it is insuch a chain, on the other hand, it will be discovered during thecollection process without its having been recorded in a remembered set.This is because any reference in such a chain is located in an objectthat is reachable and that will therefore be evacuated. And, since thatobject is evacuated, it will be scanned for references to collection-setobjects. This will cause the omitted reference to be found.

Another way is applicable particularly to collectors that employ thetrain algorithm and use scratch-pad lists, associated with respectivetrains, to list the locations where processing the collection set'sremembered-set entries revealed references to collection-set objects.When the collector scans the mutator-modified regions in accordance withthis aspect of the invention, it sometimes also omits the attendantremembered-set updating in those cases in which the references therebyfound refer to objects in the collection set. Instead, it places entriesfor those references directly into scratch-pad lists. I have recognizedthat updating the remembered set and subsequently scanning the locationthereby identified can be omitted in cases when no further referencemodification will occur.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings, ofwhich:

FIG. 1, discussed above, is a block diagram of a computer system inwhich the present invention's teachings can be practiced;

FIG. 2 is, discussed above, is a block diagram that illustrates acompiler's basic functions;

FIG. 3, discussed above, is a block diagram that illustrates amore-complicated compiler/interpreter organization;

FIG. 4, discussed above, is a diagram that illustrates a basicgarbage-collection mechanism;

FIG. 5, discussed above, is a similar diagram illustrating thatgarbage-collection approach's relocation operation;

FIG. 6, discussed above, is a diagram that illustrates agarbage-collected heap's organization into generations;

FIG. 7, discussed above, is a diagram that illustrates a generationorganization employed for the train algorithm;

FIGS. 8A and 8B, discussed above, together constitute a flow chart thatillustrates a garbage-collection interval that includes old-generationcollection;

FIG. 9, discussed above, is a flow chart that illustrates in more detailthe remembered-set processing included in FIG. 8A;

FIG. 10, discussed above, is a block diagram that illustrates in moredetail the referred-to-object evacuation that FIG. 9 includes;

FIGS. 11A and 11B, discussed above, together form a flow chart thatillustrates in more detail the FIG. 10 flow chart's step of processingevacuated objects' references;

FIGS. 12A–12J, discussed above, are diagrams that illustrate acollection scenario that can result from using the train algorithm;

FIGS. 13A and 13B together constitute a flow chart that illustrates acollection interval, as FIGS. 8A and 8B do, but illustratesoptimizations that FIGS. 8A and 8B do not include;

FIGS. 14A and 14B together constitute a flow chart that describes anadvantageous approach to memorializing the locations of newly writtenreferences;

FIG. 15 is a diagram that illustrates the addition of an entry to aremembered set; and

FIG. 16 is a flow chart that illustrates an improvement to theoperation, included in FIG. 11, of recording reference locations thathave changed because the objects containing them have been evacuated.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

As was indicated above, the present invention is not limited to garbagecollectors that employ the train algorithm. And its applicability totrain-algorithm-based collectors extends to collectors whose operationalsequences depart significantly from the sequence that FIGS. 8–11 aboveillustrate. Although the sequence there illustrated contemplatesexecuting an entire collection increment in a single interval dedicatedonly to collection activity, there are ways of spreading a collectionincrement over multiple intervals. Alternatively, most or all of thecollection increment can be performed concurrently with mutatoroperation, although, as was indicated above, this tends to be somewhatcomplex. Additionally, although the train algorithm is usuallyimplemented in a multi-generational collector, there is no reason inprinciple why collectors that use the train algorithm need to employmore than one generation.

In train-generation embodiments, it is necessary only that theevacuation policy tend to place related objects into common trains andthat the trains be checked for any strong references and reclaimed ifthey have none, as was explained above. Indeed, even in arrangements ofthe general type exemplified above in connection with FIGS. 8–11, inwhich an entire increment is performed in a single collection intervaldirected to a portion of an old generation, the sequence can differ fromthe one there illustrated. For example, FIGS. 13A and 13B (together,“FIG. 13”) depict in simplified form an approach that I prefer.

Whereas it was tacitly assumed above that, as is conventional, only asingle car section would be collected in any given collection interval,the FIG. 13 sequence contemplates collecting more than a single carduring a collection increment. FIG. 13 also depicts certainoptimizations that some of the invention's embodiments may employ.Blocks 172, 176, and 178 represent operations that correspond to thosethat FIG. 8's blocks 102, 106, and 108 do, and dashed line 174represents the passage of control from the mutator to the collector, asFIG. 8's dashed line 104 does.

For the sake of efficiency, though, the collection operation of FIG. 13includes a step represented by block 180. In this step, the collectorreads the remembered set of each car in the collection set to determinethe location of each reference into the collection set from a caroutside of it, it places the address of each reference thereby foundinto a scratch-pad list associated with the train that contains thatreference, and it places the scratch-pad lists in reverse-train order.As blocks 182 and 184 indicate, it then processes the entries in allscratch-pad lists but the one associated with the oldest train.

Before the collector processes references in that train's scratch-padlist, the collector evacuates any objects referred to from outside theold generation, as block 186 indicates. To identify such objects, thecollector scans the root set. In some generational collectors, it mayalso have to scan other generations for references into the collectionset. For the sake of example, though, we have assumed the particularlycommon scheme in which a generation's collection in a given interval isalways preceded by complete collection of every (in this case, only one)younger generation in the same interval. If, in addition, thecollector's promotion policy is to promote all survivingyounger-generation objects into older generations, it is necessary onlyto scan older generations, of which there are none in the example; i.e.,some embodiments may not require that the young generation be scanned inthe block-186 operation.

For those that do, though, the scanning may actually involve inspectingeach surviving object in the young generation, or the collector mayexpedite the process by using card-table entries. Regardless of whichapproach it uses, the collector immediately evacuates into another trainany collection-set object to which it thereby finds an externalreference. The typical policy is to place the evacuated object into theyoungest such train. As before, the collector does not attempt toevacuate an object that has already been evacuated, and, when it doesevacuate an object to a train, it evacuates to the same train eachcollection-set object to which a reference in the thus-evacuated objectrefers. In any case, the collector updates the reference to theevacuated object.

When the inter-generational references into the generation have thusbeen processed, the garbage collector determines whether there are anyreferences into the oldest train from outside that train. If not, theentire train can be reclaimed, as blocks 188 and 190 indicate.

As block 192 indicates, the collector interval typically ends when atrain has thus been collected. If the oldest train cannot be collectedin this manner, though, the collector proceeds to evacuate anycollection-set objects referred to by references whose locations theoldest train's scratch-pad list includes, as blocks 194 and 196indicate. It removes them to younger cars in the oldest train, againupdating references, avoiding duplicate evacuations, and evacuating anycollection-set objects to which the evacuated objects refer. When thisprocess has been completed, the collection set can be reclaimed, asblock 198 indicates, since no remaining object is referred to fromoutside the collection set: any remaining collection-set object isunreachable. The collector then relinquishes control to the mutator.

FIGS. 14A and 14B (together, “FIG. 14”) illustrate a way of reducing thetime required to perform FIG. 13's step 176, in which the results ofscanning “dirty” cards are recorded. As blocks 200 and 202 indicate, anyreference found in the dirty card is read to determine whether it refersto a location in the young generation. If it does, that fact ismemorialized appropriately, as block 204 indicates. The particularmemorialization technique is not of concern here, but one possibleapproach is to use an expanded card table. In such an expanded cardtable, a multi-byte array would be used to summarize the card'sreference contents. The summary may be a list of offsets that indicatethe exact locations within the card where references to young-generationobjects may be found, for instance, or it may be a list offine-granularity “sub-cards” within which references to young-generationobjects may be found. In any event, the locations of suchinter-generational references must be memorialized in some fashion, andblock 204 represents such a memorialization step.

If the result of the block-202 test is that the referred-to object isnot in the young generation but rather is (in this two-generationexample) in the old generation, then the reference's presence must stillbe memorialized, too. As was discussed above, a typical way of somemorializing the presence of an (intra-generational) reference to anold-generation object is to install an appropriate entry into aremembered set associated with the car section containing thereferred-to object.

Because of the need to avoid duplicate entries, installing aremembered-set entry can be time-consuming. To appreciate this, considerFIG. 15, which illustrates one way of adding an entry to a rememberedset's reference list. FIG. 15 depicts a memory space 206 allocated tothe remembered set's reference list as containing only sixteenreference-sized locations. Let us suppose that the reference of interestoccurs at a location whose address is 192E. To determine where to placethis address in the memory space 206 allocated to the reference list,the collector applies a hash function 208 to the address. In theillustrated example, the hash function is simply the address's fourleast-significant bits, whose hexadecimal representation is E_(H). Thecollector uses this value as an offset into the list, but it does notimmediately store the address at the list location thus identified. Itfirst reads that location's value to determine whether another addresshas already been stored there. In the FIG. 15 scenario, one already has,as its non-NULL contents indicate.

Now, if that already-stored address were itself 192E, resulting from anentry made during a previous collection interval, the collector wouldrecognize that a duplicate had occurred, and it would stop its attemptto store the value. But the already-stored address in the illustratedexample is instead 477E, so the collector proceeds to the nextreference-list location. This location, too, contains a non-NULL valuethat differs from the address to be stored. Since that location is atthe end of the list, the collector proceeds circularly to the beginningof the list and repeats the test there. Again, the location is alreadyoccupied, so it proceeds still further, and this time it finds an emptylocation.

Even though the collector has not inspected every list entry, it caninfer from encountering the empty location that the list has noduplicates of the entry to be added. Any previous attempt to store thesame value would have taken the same steps, and the collector wouldaccordingly have encountered the duplicate in attempting to find a spacefor this address. The collector therefore has simultaneously found alocation and avoided duplication.

Even with the atypically small list that FIG. 15 depicts, though, thetask of avoiding duplication has been somewhat involved. And rememberedsets can become very large indeed, particularly if the car sectioncontains a “popular” object, one to which a large number of otherobjects refer. In such a case, the task of adding a remembered-set entrycan be quite costly. But I have recognized that, if the reference is infact in the next collection set, the time-consuming operation ofinstalling a remembered-set entry is unnecessary.

To understand why this is so, recall that the remembered set's purposeis to keep track of references to objects so that the collector canultimately determine whether those objects are reachable or, instead,are garbage. Also, consider a reference that is part of a collection-setobject and refers to another old-generation object. For such an object,there are two possibilities when the next old-generation collectioninterval occurs, and neither necessitates recording the reference.

The first possibility is that the collection-set object containing thereference in question is not referred to by any object outside thecollection set (not even by any erst-while collection-set object thathas now been evacuated from the collection set because it waspotentially reachable). In that case, the object to which the referencein question refers is not reachable through that reference, so failingto record that reference in a remembered set will not prevent areachable object from being identified as such.

The second possibility is that the collection-set object containing thereference in question is indeed referred to by a reference locatedoutside of the collection set. In that case, the object referred to bythe reference in question is potentially reachable through thatreference. But there is still no harm in not having recorded the subjectreference's location in a remembered set as part of the card-scanningoperation of FIG. 14. This is because, as was explained above inconnection with FIG. 11's step 146, the necessary remembered-set entrywill be made as part of the operation of evacuating the objectcontaining the reference. If the determination of FIG. 14's step 210 isthat the reference is contained in the next collection set, therefore,there is no harm in refraining from recording the reference and, as FIG.14 indicates, simply determining whether any references remain.

If the reference is not located in the collection set, on the otherhand, then its existence must be memorialized. Even in this case,though, some savings based on collection-set membership may be realized.To appreciate this, it helps to consider precisely what is meant by the“next” collection set to which block 210 refers, and reference to FIG.13 is helpful in this context. Recall that the collection intervalalways begins with the process of scanning dirty cards, as block 176indicates. As block 178 indicates, the young generation is thencollected. As was explained above, the collection interval endsimmediately after the young-generation collection in most cases; only anoccasional collection interval also includes some old-generationcollection, and the collection set is part of only the old generation.

During collection intervals that will not include old-generationcollection, the collector knows the identity of at least one car thatwill be in the collection set for the next collection interval thatincludes some old-generation collection. There may also be enough of theinput information used by its collection-set-selection routine that itcan identify more such cars. So FIG. 14's block-210 determination ofwhether the reference is in the “next” collection set can be made,although it may be based on only part of what the complete collectionset turns out to be.

But further savings can be realized when the reference-recordingoperation of FIG. 14 occurs during a collection interval that willinclude some old-generation collecting. As blocks 212 and 214 indicate,references not in the collection set will be entered in the appropriateremembered sets if the operation of FIG. 14 is being carried out duringa collection interval that will include only young-generationcollection, not old-generation collection. If the interval will includeold-generation collection, on the other hand, the references still haveto be recorded, but those that refer to objects in the next collectionset do not have to be recorded in the remembered sets.

To understand why, first recall that the entries in a remembered setsassociated with a given car identify locations at which references toobjects in that car sections have been found, mostly during previouscollection intervals. During intervening mutator intervals, thosereferences may have disappeared. So old-generation collection includesagain scanning the locations identified by the remembered-set entries todetermine whether they still contain references to collection-setobjects. But, if FIG. 14's dirty-card scanning occurs during acollection interval in which a given car will be collected, anyreference found during the dirty-card scanning is guaranteed still to bepresent when that car section is collected. If that reference refers toa collection-set object, therefore, its location does not need to bescanned again. So it does not need to be placed in the car's rememberedset; as blocks 215 and 216 indicate, it can go directly to thescratch-pad list associated with the train to which the car containingthe reference belongs.

As block 200 indicates, the operation of scanning for referencescontinues until all references in the dirty section have been found.During most collection intervals, the procedure is finished when theyhave. In the particular intervals that will include some old-generationcollection, however, a mark can be made in an appropriate datastructure, as blocks 218 and 220 indicate, to indicate that the dirtycard has already been scanned for references to collection-set objects.That data structure can be consulted later, when the collection set'sremembered-set entries are being processed. If a collection-setremembered-set entry would otherwise cause the collector to scan alocation in that card, it refrains from doing so if the data structureindicates that the card has already been scanned.

So far, we have considered the “next” collection set to be either thecollection set that prevails for the current collection interval or, ifthe current collection interval includes no old-generation collection,the collection set that will prevail during the next collection intervalthat does include some old-generation collection. But savings inremembered-set-entry installation can also be realized during anold-generation-collection interval by knowing the identities of carscontained in a subsequent collection interval's collection set.

Recall in this connection FIG. 11's processing of references thatevacuated objects contain. When an object is evacuated, the locations ofthe references that it contains are moved, too, so areference-containing object's removal must often be accompanied byadding remembered-set entries reflecting those references' newlocations, as FIG. 11's block 146 indicates. But the block-146 operationcan be so performed as to avoid such entry addition in some cases, asFIG. 16 illustrates.

As FIG. 16's blocks 222, 224, 226, and 228 indicate, the new location ofa reference contained in an evacuated object is recorded in the summarytable if it refers to a young-generation object, and it normally-but notalways-is recorded in the appropriate remembered set if it refers to anold-generation object. If it refers to an old-generation object, thecollector first determines, as block 228 indicates, whether theevacuated object containing it is now in the collection set that willprevail during the old-generation-collection cycle after the currentone. If it is, the collector refrains from recording its location in aremembered set; reasoning similar to that set forth in connection withFIG. 14's block 210 establishes that in such a case no harm results fromthus dispensing with remembered-set-entry installation.

The present invention thus reduces the cost of remembered-setmaintenance significantly and thus constitutes a significant advance inthe art.

1. A method for updating a remembered set in a garbage collector using atrain algorithm configured to divide heap memory into a young generationand an old generation and collects a collection set of a plurality ofheap memory segments grouped into a plurality of trains duringsuccessive collection increments, the method comprising: prior toperforming a remembered set updating operation during a collectionincrement, using the garbage collector to identify the plurality of heapmemory segments in the old generation, wherein the plurality of heapmemory segments are part of a collection-set subset which is collectedin a next successive collection increment; performing the nextsuccessive collection increment; identifying a mutator-modified heapmemory segment from the plurality of heap memory segments; scanning themutator-modified heap memory segment to locate a plurality of referencestherein; performing a memorialization operation, wherein thememorialization operation excludes the plurality of references locatedin the mutator-modified heap memory segment from update in a rememberedset; placing an entry identifying a location of one of the plurality ofreferences from the remembered set into a scratchpad list associatedwith one of the plurality of trains; and processing the heap memorysegment based on the scratchpad list, wherein the entry from theremembered set is placed in the scratchpad list without being recordedin the remembered set.
 2. A method as defined in claim 1 wherein beforeobjects are evacuated during a given collection increment, thecollection-set subset for the next successive collection increment isidentified, wherein the memorialization operation is performed inresponse to an evacuation of a reference-containing object during thenext successive collection increment and includes recording in theremembered set a location of a reference contained by thereference-containing object, and wherein no locations of referenceslocated in the collection-set subset identified for the next successivecollection set are thereby recorded.
 3. A computer system comprising:processor circuitry operable to execute processor instructions; andmemory circuitry, to which the processor circuitry is responsive, thatcontains processor instructions readable by the processor circuitry toconfigure the computer system to update a remembered set in a garbagecollector using a train algorithm configured to divide heap memory intoa young generation and an old generation and collects a collection setof a plurality of heap memory segments grouped into a plurality oftrains during successive collection increments, by: prior to performinga remembered set updating operation during a collection increment, usingthe garbage collector to identify the plurality of heap memory segmentsin the old generation, wherein the plurality of heap memory segments arepart of a collection-set subset which is collected in a next successivecollection increment; performing the next successive collectionincrement; identifying a mutator-modified heap memory segment from theplurality of heap memory segments; scanning the mutator-modified heapmemory segment to locate a plurality of references therein; performing amorialization operation, wherein the memorialization operation excludesthe plurality of references located in the mutator-modified heap memorysegment from update in a remembered set; placing an entry identifying alocation of one of the plurality of references from the remembered setinto a scratchpad list associated with one of the plurality of trains;and processing the heap memory segment based on the scratchpad list,wherein the entry from the remembered set is placed in the scratchpadlist without being recorded in the remembered set.
 4. A computer systemas defined in claim 3 wherein: before objects are evacuated during agiven collection increment, the collection-set subset for the nextsuccessive collection increment is identified, wherein thememorialization operation is performed in response to an evacuation of areference-containing object during the next successive collectionincrement and includes recording in the remembered set a location of areference contained by the reference-containing object, and wherein nolocations of references located in the collection-set subset identifiedfor the next successive collection set are thereby recorded.
 5. Astorage medium containing instructions readable by a computer systemincluding memory to configure the computer system to operate as agenerational, pace-incremental garbage collector configured to: divideheap memory into a young generation and an old generation; collect acollection set of heap memory segments during successive collectionincrements using a train algorithm; prior to performing a remembered setupdating operation during a collection increment, identify a pluralityof heap memory segments in the old generation, wherein the plurality ofheap memory segments are part of a collection-set subset which iscollected in the next successive collection increment; performs the nextsuccessive collection increment; identifies a mutator-modified heapmemory segment from the plurality of heap memory segments; scan themutator-modified heap memory segment to locate a plurality of referencestherein; perform a memorialization operation, wherein thememorialization operation excludes the plurality of references locatedin the mutator-modified heap memory segment identified by the garbagecollector from update in a remembered set; place an entry identifying alocation of one of the plurality of references from the remembered setinto a scratchpad list associated with one of the plurality of trains;and process the heap memory segment based on the scratchpad list,wherein the entry from the remembered set is placed in the scratchpadlist without being recorded in the remembered set.
 6. A storage mediumas defined in claim 5 wherein: before objects are evacuated during agiven collection increment, the collection-set subset for the nextsuccessive collection increment is identified, wherein thememorialization operation is performed in response to an evacuation of areference-containing object during the next successive collectionincrement and includes recording in the remembered set a location of areference contained by the reference-containing object, and wherein nolocations of references located in the collection-set subset identifiedfor the next successive collection set are thereby recorded.